Traditional processes of drug discovery involve the screening of complex fermentation broths and plant extracts for a desired biological activity or the chemical synthesis of many new compounds for evaluation as potential drugs. The advantage of screening mixtures from biological sources is that a large number of compounds are screened simultaneously, in some cases leading to the discovery of novel and complex natural products with activity that could not have been predicted otherwise. The disadvantages are that many different samples must be screened and numerous purifications must be carried out to identify the active component, often present only in trace amounts. On the other hand, laboratory syntheses give unambiguous products, but the preparation of each new structure requires significant amounts of resources. Generally, the de novo design of active compounds based on high resolution structures of enzymes has not been successful.
In order to maximize the advantages of each classical approach, new strategies for combinatorial unrandomization have been developed independently by several groups. Selection techniques have been used with libraries of peptides (see Geysen, H. M., Rodda, S. J., Mason, T. J., Tribbick, G. & Schoofs, P. G., J. Immun. Meth. 1987, 102, 259-274; Houghten, R. A., Pinilla, C., Blondelle, S. E., Appel, J. R., Dooley, C. T. & Cuervo, J. H., Nature, 1991, 354, 84-86; Owens, R. A., Gesellchen, P. D., Houchins, B. J. & DiMarchi, R. D., Biochem. Biophys. Res. Commun., 1991, 181, 402-408), nucleic acids (see Wyatt, J. R., et al., Proc. Natl. Acad. Sci. USA, (in press); Ecker, D. J., Vickers, T. A., Hanecak, R., Driver, V. & Anderson, K., Nucleic Acids Res., 1993, 21, 1853-1856) and nonpeptides (see Simon, R. J., et al., Proc. Natl. Acad. Sci. USA, 1992, 89, 9367-9371; Zuckermann, R. N., et al., J. Amer. Chem. Soc., 1992, 114, 10646-10647; Bartlett, Santi, Simon, PCT WO91/19735; and Ohlmeyer, M. H., et al., Proc. Natl. Acad. Sci. USA , 1993, 90, 10922-10926). The techniques involve iterative synthesis and screening of increasingly simplified subsets of oligomers. Monomers or sub-monomers that have been utilized include amino acids and nucleotides, both of which are bi-functional. Utilizing these techniques, libraries have been assayed for activity in cell-based assays, in binding or inhibition of purified protein targets or otherwise.
A technique, called SURF (Synthetic Unrandomization of Randomized Fragments), involves the synthesis of subsets of oligomers containing a known residue at one fixed position and equimolar mixtures of residues at all other positions. For a library of oligomers four residues long containing three monomers (A, B, C), three subsets would be synthesized (NNAN, NNBN, NNCN, where N represents equal incorporation of each of the three monomers). Each subset is then screened in a functional assay and the best subset is identified (e.g. NNAN). A second set of libraries is synthesized and screened, each containing the fixed residue from the previous round, and a second fixed residue (e.g. ANAN, BNAN, CNAN). Through successive rounds of screening and synthesis, a unique sequence with activity in the assay can be identified. The SURFs technique is described in Ecker, D. J., Vickers, T. A., Hanecak, R., Driver, V. & Anderson, K., Nucleic Acids Res., 1993, 21, 1853-1856. The SURF method is further described in PCT patent application WO 93/04204, the entire contents of which is herein incorporated by reference.
Peptide nucleic acids have been demonstrated to be useful surrogates for oligonucleotide in binding to both DNA and RNA nucleic acids (see Egholm et al., Nature, 1993, 365, 566-568 and reference cited therein and PCT applications WO 92/20702, WO 92/20703 and WO 93/12129). Additionally peptide nucleic acids have demonstrated the ability to effect strand displacement of double stranded DNA (see Patel, D. J., Nature, 1993, 365, 490-492 and references cited therein). It is not known to prepare peptide nucleic acid libraries however or to use peptide nucleic acid monomers in combinatorial techniques.